Lymphocyte activation gene-3 (LAG-3; CD223) is a co-receptor that may function as a down-modulator of T cell activity via an interaction with MHC class II molecules. In this study, we cloned and sequenced the complete cDNA sequence of pig Lag3. Pig Lag3 cDNA contains an open reading frame (1524 bp) encoding 507 amino acids. The putative amino acid identity of pig LAG-3 with those of the human, rat, and mouse is 78%, 66%, and 67%, respectively. Pig Lag3 mRNA transcript was mainly expressed in lymphoid tissues, and was inducible upon stimulation of phytohemagglutinin (PHA). Next, we developed mouse antiserum against the extracellular domain (D1 and D2 domains) of pig LAG-3. Pig LAG-3 is detected as an approximately 67 kDa of a single species by Western blot analysis. Moreover, flow cytometry analysis confirmed that LAG-3 on pig T cells (CD3(+) cells) is inducible upon PHA stimulation. These results indicated that the structure and expression pattern of pig LAG-3 are conserved among mammalian species. (C) 2009 Elsevier B.V. All rights reserved.